Circuit for sounding an alarm when the incoming signal exceeds a given amplitude



2,995,687 NCOMING Aug. 8, 1961 r. R. MAYBERRY CIRCUIT FOR souunmc ANALARM WHEN THE I SIGNAL EXCEEDS A GIVEN AMPLITUDE Filed March 31, 1958INVENTOR.

l 3 g TED R. MAYBERRY BY Z 1G1 & mun:

United States Patent O CIRCUIT FOR souwnnic AN ALAIRM WHEN THEINCOMINGSIGNAL EXCEEDS A GIVEN AMPLITUDE Ted R. Maybeny, San Diego, Calif.,asslgnor to Ryan Aeronautical Co., San Diego, 'Calif. Filed Mar. 31,1958, Ser. No. 725,436 3 Claims. (Cl. 317-1485) The present inventionrelates generally to radar and more particularly to a radar alarm.

The primary object of this invention is to provide an alarm which isactuated by a radar echo signal whose effective strength is above apredetermined threshold, the alarm being self-resetting after the signalstrength drops below the threshold.

Another object of this invention is to provide an alarm suitable for usewith pulse type radar and which sustains an alarm indication for aperiod of time considerably in excess of the pulse duration.

Still another object of this invention is to provide an alarm circuitwhich, without modification, may be used with continuous wave radar andwill sustain an alarm indication from the time a signal-threshold isreached until the signal strength drops below the threshold.

A further object of this invention is to provide an alarm having arecovery time after an alarm has been given.

Finally, it is an object to provide a radar alarm of the aforementionedcharacter which is simple to construct and which will give generallyeflicient and durable service.

With these and other objects definitely in view, this invention consistsin the novel construction, combination and arrangement of elements andportions, as will be hereinafter fully described in the specification,particular- 1y pointed out in the claims, and illustrated in the drawing which forms a material part of this disclosure, and in which thesingle figure is a schematic wiring diagram of the alarm circuit.

Referring now to the circuitry shown in the drawing, the alarm comprisesbasically a -two-stage D.C. amplifier using a first transistor and asecond transistor 20. The first transistor 10 has a base electrode 12, acollector electrode 14 and an emitter electrode 16, the secondtransistor 20 similarly having a base electrode 22, a collectorelectrode 24 and an emitter electrode 26, the term electrode beingomitted hereinafter. The input 30 is connected directly to the base 12through a resistor 32, while the collector 14 is connected to thebase'22 through a coupling resistor 34. The emitter 16 is connected tothe emitter 26 and thence to ground through a resistor 36.

* The collector 24 is connected to one end of a relay coil 38, the otherend of the coil being connected to the positive terminal 40 of abattery, or similar source of DC. voltage. 1 The positive terminal 40also provides voltage, through a load resistor 42, to the collector 14of transistor 10, the negative terminal 44 of the battery beingconnected to ground. The collector 24 is connected back to the base 12through a feedback circuit comprising, in series, a coupling capacitor46, a decoupling diode 48 and a resistor 50, the feedback circuit beingcoupled to ground through a bypass diode 52 connected between thecapacitor 46 and diode 48. The relay contacts 54 are wired in a suitablemanner to an alarm indicator 56, which is actuated by the relay. Thealarm indicator 56 may be of any suitable type such as an audible orvisual indicator.

The-D.C. amplifiercomprising the transistors 10 and 20, maintains aconstant current in the circuit and through the relay coil 38 andresistor 36. This current is sufiicient to operate the relay contacts 54and is necessary to cs- Patented Aug. 8, 1961 through resistor 36,resulting in the turning on of transistor 10. When a positive pulse typesignal is received at the input 30, that portion of the signal, which isgreater in amplitude than the threshold bias voltage, is amplified andcauses a considerable drop in current through the relay coil 38,sufficient to cause the relay contacts 54 to operate, so actuating thealarm indicator 56. Since the normal radar signal pulse is of extremelyshort duration, it is necessary to sustain the alarm conditionsufilciently long to obtain a useful indication. This is accomplished bythe positive feedback circuit which feeds amplified signal current fromthe output collector 24 back to the input base 12 through capacitor 46,diode 48, and resistor 50, and maintains an effective signal for aconsiderably longer period than the actual pulse, as hereinafterdescribedin detail. The capacitor 46 passes the A.C. feedback butprevents the DC. circuit voltage from feeding the input sufliciently tocause a false alarm indication. The diode 48 is a silicon diode which isan open circuit for small signals, so that there is normally nofeedback, said diode being a relatively high impedance for low voltages,less than 0.6 volt, in the forward direction, thus decoupling thefeedback for small disturbances and preventing feedback or oscillationdue to noise. With the advent of a large amplified signal voltageappearing on the collector 24, the diode 48 conducts and allows feedbackto the input base 12. The bypass diode 52 provides a quick discharge toground for the capacitor 46, thereby resetting the circuit and reducingthe recovery time of the circuit. The cathode of diode 52 is positivewith respect to the anode thereof when the first transistor 10 isconducting, due to an incoming signal. As soon as the capacitor 46 ischarged, the first transistor 10 stops conducting, or is turned off, andthe second transistor 20 is turned on. Diode 52 is now biased in theforward direction by the negative pulse from the collector 24 oftransistor 20, which is turning on, so allowing current to flow throughsaid diode and furnishing a low impedance discharge path. This resettingaction is automatic as soon as the signal strength drops below thethreshold and is thus effective for true alarm indications or for falsealarm indications caused by noise or interference. The feedback issustained as long as the second transistor 20 is turned off, the timebeing governed by the charging time of capacitor 46.

Initially, when there is insuflicient or no input signal at input 30,the current through relay coil 38 is high enough to energize the relayand close contacts 54 in one direction so that the indictor 56 indicatesno signal. When sufiicient signal is received at input 30, the amplifiedportion is added to the current in the relay coil, the summationactually causing the current in the coil to drop and release contacts54, which return to the other position and actuate indicator 56 todenote an alarm condition. When a steady or continuous wave signal withamplitude greater than the threshold voltage established by resistor 36is received at the input 30, the relay contacts 54 are immediatelyactuated to operate the alarm indicator 56 and the alarm condition issustained until the input signal voltage drops below the threshold. Thusa steady alarm indication is provided during the entire time that asignal of sufficient intensity is being received. Without diode 48, ifthe circuit is operating above the threshold for any length of time, anundesirable oscillation is likely to occur. However, the diode 48decouples the feedback circuit before any undesirable oscillation canbuild up. Again, the bypass diode 52 provides a quick discharge forcapacitor 46, so that the circuit can recover for a further signal. x,

In the feedback circuit, the initial state is as follows: input voltageat 30 is zero, emitter 16 and collector l4 currents are zero, base 22current through resistors 42 and 34 is at maximum, collector 24curnentthrough relay coil 38 is at maximum, .emitter 26 current throughresistor 36 is at maximum; capacitor 46 is uncharged and the voltagedrop developed by resistor 36 constitutes negative bias. for transistor10. When a positive signal is applied to input 30 and is of sufficientamplitude to over.- come the negative bias on emitter 16, collector-14cond-ucts, causing a large drop in voltage at said collector andreducing base 22 current and collector 24 current to zero. Capacitor 46now charges through diode 48, resistor 50, relay coil 38 and base 12,thus providing suflicient current to said base to maintain transistor inits on 'or conducting state. The current at base 12 is present untilcapacitor 46 has charged and is independent of any signal current atinput 30. If a signal current continues at input 30, the circuit remainsin the same state with transistor 10 on and transistor otf. However, ifthe input signal ceases while capacitor 46 is charging, the current atbase 12 gradually drops until it is insufficient to hold the collector14 current, which returns to its initial value and permits transistor 20to be turned on again. This last condition initiates discharge ofcapacitor 46 through diode 52 and completes a cycle of operation.

The circuit may be adjusted so that the normal inputv noise voltage,present on terminal in the absence of a radar echo signal, is below theselected threshold. In this manner, the alarm indicates only objects ortargets which may be of interest and which cause an echo signal ofsufficient intensity to pass the circuit threshold. It is emphasizedthat the circuit is sensitive to pulse or continous wave inputs and toA.C. or slowly changing DC, the amplitude and duration being theimportant factors rather than specific signal'characteristics.

The operation of this invention will be clearly comprehended from aconsideration of the foregoing descrip- -tion of the mechanical detailsthereof, taken in connection with the drawing and the above recitedobjects. It

i will be obvious that all said objects are amply achieved by thisinvention.

It is understood that minor variation from the form of I the inventiondisclosed herein may be made without departure from the spirit and scopeof the invention, and

that the specification and drawing are to be considered as merelyillustrative rather than limiting.

I claim:

1. A twotransistor circuit having an alarm state when an input signalexceeds a predetermined amplitude, and having a non-alarm state in theabsence of an input signal that exceeds said predetermined amplitude,said circuit maintaining said alarm in its alarm state for a selectedinterval of time, comprising: a first transistor having an inputterminal; a second transistor having an output terminal; a source ofpotential having a negative terminal; means, comprising connectionsbetween said source and said first transistor, for causing saidtransistor to be nonconductive in said non-alarm state; means,comprising connections between said source and said second transistor,for causing said second transistor to be conductive in said non-alarmstate; a load circuit connected to said output terminal of said secondtransistor, said load circuit comprising the coil of an alarm relay,whereby when said second transistor is conductive it maintains a currentfiow through said load circuit, said current flow maintains said alarmrelay in said circuits non-alarm state; means for causing said firsttransistor to become conductive and for simultaneously causing saidsecond transistor to become non-conductive in the presence of apositive-going input signal that exceeds said predetermined amplitude,the circuit thus switching to its alarm state; a positive feedbackcircuit connected between said output terminal of mit a positivefeedback signal to said first transistor,

whereby feedback signal tends to maintain said abnormal state; rneansfor discharging said capacitance when the normal state is restored, saiddischarging means comprising a diode connected between said negativeterminal I and said capacitance for transmitting a signal through saidcapaclitance in a direction opposite that of the feedback srgna 2. Atwo-transistor circuit for energizing an alarm when an incoming signalcomprising a single incoming radar pulse, a train of pulses, or a radarwave exceeds a predetermined amplitude, said circuit maintaining saidalarm in its energized state for a selected interval of time,comprising: a first transistor having an input terminal; a secondtransistor having an output terminal; a source of potential having anegative terminal; means, comprising connections between said source andsaid first transistor, for causing said transistor to be non-conductivein said non-alarm state; means, comprising connections be tween saidsource and said second transistor, for causing said second transistor tobe conductive in said non-alarm state; a load circuit connected to saidoutput terminal of said second transistor, said load circuit comprisingthe winding of an alarm relay and a load resistance; biasing means,activated by said load circuit, for controlling the bias of said firsttransistor; means for activating said first transistor and cutting offsaid second transistor in the presence of said input signal; a positivefeedback circuit connected between said output terminal of said secondtransistor and said input terminal of said first transistor, saidfeedback circuit comprising a series-connected capacitance, and adecoupling diode poled to transmit a positive feedback signal to saidfirst transistor; and means for discharging said capacitance, saiddischarging means comprising a diode connected between said negativeterminal and said capacitance, said discharging diode being poled totransmit a signal through said capacitance in a direction opposite. thatof the feedback signal.

3. A two-transistor circuit for energizing an alarm and maintaining saidalarm in its energized state for a selected interval of time, when aninput comprising a single incoming radar pulse, 'a train of radarpulses, or a radar wave exceeds a predetermined amplitude, said circuitcomprising: a first normally non-conductive transistor having a base, acollector, and an emitter; a second normally conductive transistorhaving a base, a collector, and an emitter; a source of potential havinga positive terminal and a negative terminal; a load circuit comprisingthe winding of an alarm relay connected between said positive terminaland said collector of said second transistor, and a load resistanceconnected between said emitter of said second transistor and saidnegative terminal; a biasing direct connection between said emitter ofsaid first transistor and said emitter of said second transistor,whereby when said normally conductive secondtransistor maintains acurrent flow through said' load circuit, said current flow maintainssaid alarm relay in a normal nonalarm state, and biases-said firsttransistor to its nonconductive state; a load resistance connectedbetween said positive terminal and said collector of said firsttransistor; a biasing resistance connected between said collector ofsaid firsttransistor and said base of said second transistor, wherebythe presence of said input produces an abnormal state wherein said firsttransistor is conductive, said biasing resistance cuts off said secondtransistor, and said biasing connection removes the bias from said firsttransistor and permits it to remain conductive; a positive feedbackcircuit connected between the collector of said second transistor andthe base of I capacitance upon the return of said circuit to its normal,

state, said discharging means comprising a diode connected between saidnegative terminal and the juncture between said decoupling diode andsaid capacitance, said discharging diode being poled to transmit asignal through said capacitance in a direction opposite that of thefeedback signal, whereby when said circuit returns to its normal state,said capacitance may be discharged.

References Cited in the file of this patent UNITED STATES PATENTS WiseSept. 30, Harris Sept. 20, Pinckaers Mar. 23, Walz June 3, Mitchell Aug.19, Pinckaers Mar. 24,

FOREIGN PATENTS Great Britain July 19,

